Fabrication of steel mats



0V. l0, 1970 w, ERNST 3,539,752

FABRICATION oF STEEL MATS Filed Aug. 3, 1966 i mid-kh-A-fzi-/H fr? (f/ MWl, Ul 06 /TA/L RAT/f ATTORNEY Patented Nov. 10, 1970 3,539,752FABRICATION F STEEL MATS Walter Ernst, Strump, Post Osterath, Germany,assignor to Bau-Stahlgewebe G.m.b.H., Dusseldorf-Oberkassel, Germany, acorporation of Germany Filed Aug. 3, 1966, Ser. No. 569,873 Claimspriority, application Germany, Aug. 12, 1965,

B 83,2 Int. Cl. B23k 9/00; C21d 1/00 U.S. Cl. 219-56 3 Claims ABSTRACT0F THE DISCLOSURE The present invention relates to means for and amethod of fabricating steel or the like metal mats, especially of thetype used as inserts in reinforced concrete construction and consistingof sets of spaced and intersecting longitudinal and cross bars or rodsconnected by electric spot welding at their intersection points.

In the fabrication of rectangular steel mats or units of this type,consisting of relatively long longitudinal bars intersected by shortercross bars, it is customary to connect the bars at their intersectionpoints by electric resistance spot welding by the aid of suitablewelding devices enabling the welding of the intersection points along across bar by a single operation. Since the known welding devices arenormally operated at room or ambient temperature, an excessively hightemperature obtains during the instant of welding in the vicinity of theintersecting areas or weld spots of any two crossing adjoining bars,said temperature decreasing relatively rapidly upon termination of thewelding operations as a result of the direct contact of theintersections with the surrounding atmosphere. This relatively rapidtemperature change or cooling of the welding joints has been found tofrequently result in brittle or inferior weld joints and, in turn, indefective and unsatisfactory mats or construction units. Where a singlesuch defective unit is used in a known manner in an assembly comprisinga large number of units with their marginal areas in overlappingrelation, to produce a cornposite mat structure of relatively largersize, a single inferior unit will result in the defectiveness of theentire assembly or reinforcing structure.

Accordingly, an important object of the present invention is theprovision of a novel method of an apparatus for the spot welding ofsteel or the like mats of the referred to type by which the foregoingand related defects and diiiiculties, in particular the production ofbrittle weld joints, are subtsantially eliminated or minimized.

The invention, both as to the foregoing and ancillary objects, as wellas novel aspects thereof, will be better understood from the followingdetailed description, taken in conjunction with the accompanying drawingforming part of this specification and in which:

FIG. l is a schematic cross-sectional view of welding apparatus suitablefor the carrying into effect of the irnproved spot welding method forthe production of steel or the like mats or structures in accordancewith the invention;

FIG. 2 is a graph explanatory of the function and operation oftheinvention;

FIG. 3 is a diagrammatic view similar to FIG. 1, showing a modifiedwelding apparatus for the practicing of the invention; and y FIG. 4illustrates still another welding apparatus for the carrying into effectof the invention.

Like reference characters denote like parts in the different views ofthe drawing.

With the foregoing object in view, the invention involves generally theprovision of a novel method of and means for the welding of steel or thelike mat structures of the referred to type comprising the steps ofpre-heating the bars to be welded in the vicinity of the intersectionpoints to a predetermined temperature above ambient or room temperature,spot welding the bars at said points at elevated temperature, andretarding the cooling of the joints produced to room temperature, tosubstantially prevent the occurrence of any abrupt or steep temperaturechanges, prior to and following the Welding operations proper.

In other words, according to the present invention the metal at and nearthe crossing points of the bars to be welded is subjected to apredetermined heating cycle comprised of a iirst or heating period, togradually raise the metal to a predetermined temperature above ambientor room temperature, followed by a second or cooling period, to againgradually reduce the temperature to room temperature according to acontinuous time function, the actual welding of the bars being effectedduring the brief interval or transition from said heating to saidcooling period. As a consequence, excessive sudden temperature changesor gradients between the weld spot proper and the surrounding metal oratmosphere are substantially eliminated or minimized, thus enabling agradual re-crystallization of the molten rnetal and formation of aneflicient weld joint substantially devoid of brittleness and otherundesirable characteristics.

In carrying into effect the invention, the bars to be welded may becontinuously passed through a heating field or space of graduallyincreasing and subsequently decreasing temperature distribution, thevariation of the temperature preferably following a known e-function.For ipractical purposes, the maximum temperature at which welding iseffected may be about 450 C. in the case of steel bars or rods beingwelded.

As a result of the gradual heating of the bars, or of the metal at theintersecting points, such as by passing the same through a heating fieldof gradually varying temperature distribution, the metal to be joined isprevented from being subjected to any sudden or steep ternperaturechanges during welding. There is thus ensured the attainment of reliableand efficient weld joints substantially devoid of brittleness and otherdefects.

The heat required for the pre-heating of the crossing points prior towelding may be supplied by any suitable heating source, such as in theform of a hot air heating aggregate designed and arranged to enableready operation of the welding devices at the intersection points, onthe one hand, as well as the unimpeded feeding of the structures to bewelded through or past said devices.

As will be appreciated from the foregoing, an important feature of theinvention, aside from the local preheating of the bars at and in thevicinity of the crossing points, consists in the weld joints beingpassed, during the further and continued feeding of the mats, through aheating iield or space of gradually decreasing temperature distributionproduced by means disposed behind the welding devices as viewed in thefeeding direction. In other words, according to this modification forthe carrying into effect of the invention, the heating source or sourcesfor the pre-heating `of the bars prior to welding, on the one hand, andfor the retarded cooling after welding, on the other hand, arestationary and preferably form a unitary structure together with thewelding devices, While the mats to be welded are fed continuously and/orstep-by-step for the sequential welding of the cross bars to thelongitudinal bars, in the manner as will become further apparent as thedescription proceeds.

The heating field, to effect a gradual cooling of the welding joints,may be derived from the same heat source which serves for thepre-heating of the bars. In such a case, the heat so to speak flows overthe crossing points, tovheat the same to the desired temperature, andthereafter passes into the heating eld or channel.

In such an arrangement, the thermal flux through said channel does notaffect the longitudinal bars in the vicinity of the crossing points, butrather at a point following the welding device in the feed direction. Bythis deviation of the thermal flux, the longitudinal bars Will remainsuiciently heated at a certain distance from the welding position.During the feeding of the mats, the heated air is discharged throughsieve-like openings or apertures in the heating channels, to act on thepassing cross bars, in the manner further described in reference to andshown by the drawing.

Alternatively, it is possible to utilize separate heat sources for thepre-heating of the bars and maintenance of a heating eld, to` graduallyheat and retard, respectively, the cooling of the welding spots. Again,hot air under pressure is advantageously used for this purpose beingcirculated through suitable heating channels or conduits, in such amanner as to increase the temperature in the direction towards and todecrease it in the direction away from the welding position. There isthus insured a desired maximum temperature in the Vicinity of thewelding electrodes, being below the melting or recrystallizationtemperature of the metal, such as about 450 C. in the case of steel barsor rods, as pointed out hereinabove.

In practice, it has been found advisable to provide for an asymptoticrather than a linear temperature variation or decrease of thetemperature, that is, following an e-function, to ensure a gradualcooling free from sudden or abrupt temperature changes.

Finally, the intersection points may be heated by a single heat sourcein the form of one or more hot air jets or streams with the intensitythereof being controlled, preferably by means synchronized with thewelding operation, to result in the desired heating and cooling cyclesand welding during the intervals between the rising and fallingtemperature periods, in the manner as will become further apparent asthe description proceeds in reference to the drawing.

Referring more particularly to FIG. 1, the mat to be welded comprises aplurality of longitudinal bars (only one of which appears in thedrawing) intersected by cross bars 11 Welded or to be welded,respectively, to the bars 10 at the intersection points. The matstructure is fed in the direction of the longitudinal bars 10', asindicated by the arrow A, past or through a stationary welding andheating device. In the drawing, only a single device is shown for thebar 10 in the position for Welding of the left-most cross bar 11, itbeing understood that similar welding devices (not shown) are providedfor the remaining longitudinal bars of the mat for the simultaneouswelding of all the intersection points of a single cross bar at a time,in a manner readily understood.

In the embodiment for the carrying into effect of the invention as shownby FIG. l, the areas at and surrounding the intersection points of thebars are pre-heated tothe desired temperature above room or ambienttemperature by means of hot air streams or jets applied through a pairof nozzles 12 arranged at right angle to one another and directedtowards the crossing or intersection point being welded, as indicated bythe arrows C in the drawing. As a consequence, welding of the graduallyheated bars by means of a pair of conventional resistance weldingelectrodes 13, Ioperable in the directions indicated by the doublearrows B, occurs above room temperature, such as 450 C. in the referredto example, to thereby avoid sudden or abrupt temperature changes duringwelding. In order to retard the cooling of the joints being produced, orto ensure a gradual recrystallization of the molten metal, the same heatsource is utilized for the production of a heating eld or space actingupon the welding joints after passing the welding position or electrodes13. This is achieved, in the example shown, by causing the air streamsC, upon passing or flowing over the intersection point, to pass througha pair Iof U-shaped heating channels 14 disposed parallel to and oneither side of the cross bar 11, the walls of the inner legs of saidchannels facing the bar 11 being provided with sieve-like apertures 16,to direct the air towards said bar in a plurality of small jets asindicated by the arrows D. The apertures 16 are so designed as toprovide a heating field or space effective on said bar or welding jointof gradually decreasing temperature in the direction away from thewelding position as viewed in the feed direction A. If desirable, theair may be re-heated by the provision of suitable auxiliary heatingmeans, such an additional hot air stream being passed through the space15 between the U-legs of the channels 14.

In an arrangement of the foregoing type, if the intensity of the jets orstreams C is such as to result in a continuous or uninterrupted owthrough the channels 14'and out of the apertures 16, the resultanttemperature distribution or decay may be suicient to provide for therequired retardation of the cooling of the welding joints to roomtemperature to prevent brittleness and other defects of the joints. Bythe provision for a staggered or gradual increase of the diameter of theapertures |16 in the direction towards the welding position or theelectrodes 13, the temperature distribution may be controlled to suitany existing conditions or requirement. The latter procedure may be ofspecial advantage Where the intensity of the streams C is such as toresult in a temporary air storage within the channels 14 prior to thedischarge by the apertures 16.

As is understood, the welding devices and heating apparatus of FIG. 1may be in the form of separate units or preferably combined into aunitary structure by the casings or channels 14 providing a support forthe electrodes 13, as indicated schematically at y-z in the drawing.

The mat being welded may be fed continuously at a proper speed andwelding effected by the operation of the welding electrodes 13 at theinstant of an intersection point of the bars passing the weldingposition. If desired, the mat may be briefly arrested in the weldingposition, to suit existing operating conditions or requirements.

FIG. 2 illustrates the preferred temperature cycle or variationsubstantially following an e-function before (ascending branch a) andafter welding (descending branch b), welding by the electrodes 13 beingeffected at the instant of the cycle which starts at t1 and ends at t2.Tm represents the maximum pre-heating temperature, that is, about 450 C.in the example mentioned, while TR represents room temperature.

According to the modification shown by FIG. 3, two sets of heatingchannels or conduits 18' and 18 are provided arranged in back-to-backrelation to the welding device, to produce a pair of leading and laggingheating elds, in respect to the feeding direction, of increasing anddecreasing temperature distribution, respectively, as indicated at x-y-zin the drawing, to again result in a heating cycle of the intersectionpoints as shown by FIG. 2 during passage through the device in thedirection of the arrow A.

Finally, in the FIG. 4 modification according to which the longitudinalbars 10 are fed step-by-step at right angle to the cross bar 11 or toplane of the drawing, the welding electrodes 13a-13d for thesimultaneous welding of all the longitudinal bars .10 of a mat to acommon cross bar 11 are all connected to a hot air supply or stream byway of nozzles 12a-12d, respectively, said stream being periodicallycontrolled in intensity by a valve or the like control device 20, toagain result in a heating cycle of the intersection points `as shown `byFIG. 2. In this case, the operation of the welding electrodes 13a-13d isadvantageously synchronized both with the operation of valve Z and thefeeding mechanism for the mat 10, 11, to result in automatic welding ofthe mat in a manner readily understood.

In the foregoing the invention has been described in reference to specicillustrative devices. It will be evident, however, that variations andmodications, as well as the substitution of equivalent parts or devicesfor those shown herein for illustration, may be made in accordance withthe broader purview and spirit of the invention. The speciication anddrawing are accordingly to be regarded in an illustrative rather than arestrictive sense. t

I claim:

1. Apparatus for the electric spot welding of a mat structure made up ofsets of mutually spaced and intersecting steel bars comprising rstheating means to raise said bars in the vicinity of an intersection tobe welded to an elevated temperature substantially in excess of ambienttemperature, electric spot welding means to weld said bars at saidintersection while at said elevated temperature, and further meansindependent of said first heating and said welding means and adjoiningsaid welding means, to maintain a heat tield having a temperaturedistribution along a predetermined path gradually decreasing from saidelevatedV to ambient temperature, to effect cooling of the weldedintersection to ambient temperature by passing through said eld alongsaid path.

2. ,In welding apparatus for the spot welding of a mat structureaccording to claim 1, wherein said structure is fed with theintersections to be welded being moved towards and into relativeoperative position to and past said welding means, said heating meansbeing comprised of at least one hot air heater applying a stream of hotair under pressure to the intersection to be welded, and said furthermeans being comprised of means to direct the air upon flowing over saidintersection to establish a heating field of gradually decreasingtemperature distribution traversed by said intersection upon passingsaid welding means.

3. Apparatus for the spot welding of mat structures according to claim1, wherein said structure is fed with the intersections to be weldedbeing moved into relative operative position to and past said weldingmeans, said heating and further means being each comprised of meansproviding a hot air heating space adjoining the path of saidintersections preceding and following, respectively, said welding means,to gradually heat and cool an intersection prior to and after Welding,respectively.

References Cited UNITED STATES PATENTS JOSEPH V. TRUHE,

L. A. SCHUTZMAN,

Primary Examiner Assistant Examiner U.S. Cl. X.R. 28-497; 148-427;2l9-58, 61, 124

